1. Field of the Invention
The invention relates to the field of glazings that diffuse light and are intended for lighting rooms.
2. Discussion of the Background
It has been known for a long time to transform a glass that is transparent and with parallel faces like a float glass into glass that diffuses light. The two most usual techniques are obscuring by sand blasting and etching by a paste having a hydrofluoric acid base. On the other hand, the technique of rolling flat glass with rollers to reproduce the surface structure of the rollers on the surface of the glass, by thus producing a patterned glass, is also well known. The two preceding families of production methods each make it possible to obtain glazings that diffuse light, but both exhibit certain drawbacks. Thus, the treatment methods of the float glass are very aggressive and "damage" the surface of the glass whose mechanical strength is reduced. Moreover, the surface has lost the characteristics which are those of a "fire polish" and it becomes easily soiled and difficult to maintain. In regard to the patterned glasses, the performances that they provide are often mediocre and always very different from those of diffusing glasses obtained by treatment of float glass.
Of the two techniques of the prior art which make it possible to obtain a plane glazing with a diffusing surface, the most common is sandblasting of a plane glass polished on both its faces, such as a float glass. Sandblasting techniques are used a great deal in multiple industries, they are used in closed chambers in which a pump produces the circulation of an abrasive powder (sand of defined grain size, corundum . . . ) in suspension in pressurized air. This sandblasting make it possible to treat the surfaces by attacking them to a more or less great depth, the result obtained is variable, from a simple cleaning of the surface to a restructuring in depth. Applied to a plate of polished glass, the method makes it possible to obtain an evenly obscured surface which, according to the depth of its raised patterns, more or less diffuses the light both in transmission and in reflection. At times, for various technical or aesthetic reasons, it is desired to keep a good transparency in certain zones of the glass plate. It is sufficient to achieve the result of protecting the relevant part with an elastomer film before inserting the plate into the chamber. The products used are, for example, sheet neoprene or latex deposited by brush. The concerned zone will appear intact after the separation of the elastomer. The purpose of such transparent zones in the center of totally diffusing surfaces can be to make it possible for an observer who brings his eye close to the glazing to see perfectly from the other side without being observed himself. It is also possible, by the same method, to obtain varied aesthetic effects.
The sandblasting technique is very simple to use and it makes it possible to obtain with a great versatility the reserves that are desired. The products obtained, however, exhibit two serious drawbacks which greatly limit the use of the sandblasting technique: sandblasted glazings are embrittled and very easily soiled. The greater brittleness of the sandblasted glass is noted if its resistance is compared to the bending (four point method) of the float glass with a base of that of an identical sandblasted sample, it is found that the value of the stress at the break point goes from 200 to 50 MPa. A practical drawback resides in the great facility with which the sandblasted glasses are soiled. Fingerprints, in particular, leave marks on their surface, which is very difficult to clean. The only means is to use powerful solvents and to act mechanically by brushing vigorously.
Etching, the other means used to make a float glass diffusing, makes it possible to overcome the two preceding drawbacks. The techniques of acid frosting are well known to specialists. More or less viscous solutions, pastes, are marketed. They all have a hydrofluoric acid base to which are added various chemical products, such as, for example, ammonium fluoride and fillers. The difficulty of this technique resides in its use: the products involved are very active and it is very difficult to make them act evenly. For this purpose, it is possible to use silk-screening techniques through a sieve of polyamide textile placed on the surface of the glass while the paste is displaced over the upper face of the sieve with a scraper. Another technique, that of French patent FR 1 503 587, proposes using an aggressive solution in the form of an aerosol to obtain the condensation of sized droplets on the glass. The product obtained is even and does not exhibit the drawbacks of the sandblasted product in particular, if the use of the method is carefully done, the mechanical strength of the product is of the same order--and even at times better--than that of the base glass. On the other hand, the surface state obtained is such that fingers generally leave no mark there.
However, the acid frosting technique shows two types of drawbacks. On the one hand, it poses problems relative to the environment, and on the other hand, it does not make it possible to obtain easily a composite glazing, diffusing for some parts of its surface, transparent for the remainder. Concerning the latter point, the protections that could be placed on the surface of a float glass to prevent the frosting of the zone concerned exhibit excess thicknesses that are troublesome for silk-screening and, on the other hand, chemical solutions are so aggressive that they penetrate under the protections and prevent obtaining clear boundaries at the border of the two zones. The dangers of the industrial use of hydrofluoric acid are well known, both for men and for the environment.
Methods making it possible to leave a given impression on hot glass have also been used to obtain special optical effects on the finished product.
U.S. Pat. No. 1,718,824 thus proposes, thanks to a technique of casting on a table, impressing in the surface of the glass a diffusing background thanks to a structure comprising holes which make the glazing translucent with a "frosted" surface. This diffusing background is interrupted by grooves with a V-shaped section which refract the light. This structure of the surface of the glass is the impression left by that of the table on which the glass is distributed before being rolled by a roller which leaves it an unstructured surface on the other side.
A more recent technique which relates to glass or plastic plates and which refers to the rolling of the glass between two rollers is described in patent FR 1 281 723. Its principle is to obtain a rippled effect by impressing on each of the faces of the glass a regular pattern consisting of regular rows of small conical prisms or circular cones in relief and/or recessed, the originality of the technique consisting in placing the rows of one face so that they make a small angle with the rows of the other.
The techniques described in the two preceding patents either give no detail on the diffusing structure, like the first which speaks of "the usual manner" of producing the "frosted" surface, or else show, like the second, a very rough structure where the pitch of the circular cones is on the order of the thickness of the glass plate, which is essential, moreover, to produce the indicated geometric figures.